1. Field of the Invention
The present invention relates to an image forming apparatus that forms a pattern on a supporting medium and performs correction of an image quality by using the pattern.
2. Description of the Related Art
The image forming apparatus that forms a pattern on a supporting medium and performs correction of an image quality by reading a formed pattern is known. A color image forming apparatus forms an image of a plurality of color components at each prescribed forming position, and outputs each image in a state of being superposed on a recording sheet. At this time, when a position forming the image of each color component is deviated, this is regarded as a color misregistration by a user. The color misregistration is included in the image quality. Therefore, in order to make the color misregistration unobtrusive, an image forming position of each color component is corrected by forming a color misregistration pattern and reading its position. Alternatively, in order to correct a change in density of each color component with time, there is an image forming apparatus that prepares a density pattern, reads and corrects the density of the formed pattern. The density of each color component is included in the image quality. Also, there is an image forming apparatus that forms a position, where the color misregistration pattern is formed, at opposite end portions in a width direction, when a direction orthogonal to a transfer direction of the recording sheet is set as the width direction (e.g., see Japanese Unexamined Patent Application No. 2004-109876).
In order to maintain an excellent image, an interval of correction is preferably set shorter. However, a considerable processing time is required for forming and detecting the pattern. Therefore, an opportunity of performing correction is set not to obstruct an image formation (print), being an original function of an apparatus, as much as possible.
The timing of the correction is, for example, performed in many cases just before starting page(s) of print (print job) or just after ending the print job. This is because a total processing time is shortened by interposing a preparation step before image formation and a subsequent step after image formation, between pattern formation and an image formation to be outputted. Also, there is proposed the image forming apparatus that forms the image so as to interpose the color misregistration pattern between pages in mid-course of a print job (e.g., see Japanese Unexamined Patent Application No. 2006-171352).
In addition, there is proposed the image forming apparatus that sets a width of the supporting medium larger than a maximum width of the image by having a margin, and forms the color misregistration pattern at an area outside the maximum image width (e.g., see Japanese Unexamined Patent Application No. 2005-99716).
However, when the pattern is formed before starting the print job, start of the print job is delayed. When the pattern is formed after the print job is ended, start of the next print job is delayed. When the pattern is formed between pages in mid-course of the print job, an interval between pages must be increased, thereby delaying the print speed. Further, in an apparatus using an intermediate transfer medium as the supporting medium, the pattern is not completely cleaned and is remained, thus causing a backside of the subsequent recording sheet to be stained. In the apparatus that forms the pattern in the area outside the maximum image width, decrease of the print speed or the delay of the start of the print job does not occur, compared to the aforementioned apparatus. However, the width of the supporting medium must be set sufficiently larger than the maximum image width, and accordingly, the width of each part of an image forming section must be set larger than the maximum image width. Therefore, the apparatus is inevitably upsized. Also, along with a larger width, there is a possibility of inviting an increase of cost.
Each method has both merit and demerit. A method having both of them is further desired.